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Message-ID: <e6be9a9c-2ff0-11f1-e10b-cd2f54820622@ovn.org>
Date: Thu, 15 Oct 2020 13:04:37 +0200
From: Ilya Maximets <i.maximets@....org>
To: Eelco Chaudron <echaudro@...hat.com>,
Ilya Maximets <i.maximets@....org>
Cc: netdev@...r.kernel.org, dev@...nvswitch.org, bigeasy@...utronix.de,
jlelli@...hat.com, kuba@...nel.org, pabeni@...hat.com,
davem@...emloft.net
Subject: Re: [ovs-dev] [PATCH net v2] net: openvswitch: fix to make sure
flow_lookup() is not preempted
On 10/15/20 12:54 PM, Eelco Chaudron wrote:
>
>
> On 15 Oct 2020, at 12:27, Ilya Maximets wrote:
>
>> On 10/15/20 11:46 AM, Eelco Chaudron wrote:
>>> The flow_lookup() function uses per CPU variables, which must not be
>>> preempted. However, this is fine in the general napi use case where
>>> the local BH is disabled. But, it's also called in the netlink
>>> context, which is preemptible. The below patch makes sure that even
>>> in the netlink path, preemption is disabled.
>>>
>>> In addition, the u64_stats_update_begin() sync point was not protected,
>>> making the sync point part of the per CPU variable fixed this.
>>>
>>> Fixes: eac87c413bf9 ("net: openvswitch: reorder masks array based on usage")
>>> Reported-by: Juri Lelli <jlelli@...hat.com>
>>> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
>>> ---
>>> v2: - Add u64_stats_update_begin() sync point protection
>>> - Moved patch to net from net-next branch
>>>
>>> net/openvswitch/flow_table.c | 56 +++++++++++++++++++++++++-----------------
>>> net/openvswitch/flow_table.h | 8 +++++-
>>> 2 files changed, 39 insertions(+), 25 deletions(-)
>>>
>>> diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
>>> index e2235849a57e..d90b4af6f539 100644
>>> --- a/net/openvswitch/flow_table.c
>>> +++ b/net/openvswitch/flow_table.c
>>> @@ -172,7 +172,7 @@ static struct table_instance *table_instance_alloc(int new_size)
>>>
>>> static void __mask_array_destroy(struct mask_array *ma)
>>> {
>>> - free_percpu(ma->masks_usage_cntr);
>>> + free_percpu(ma->masks_usage_stats);
>>> kfree(ma);
>>> }
>>>
>>> @@ -196,15 +196,15 @@ static void tbl_mask_array_reset_counters(struct mask_array *ma)
>>> ma->masks_usage_zero_cntr[i] = 0;
>>>
>>> for_each_possible_cpu(cpu) {
>>> - u64 *usage_counters = per_cpu_ptr(ma->masks_usage_cntr,
>>> - cpu);
>>> + struct mask_array_stats *stats;
>>> unsigned int start;
>>> u64 counter;
>>>
>>> + stats = per_cpu_ptr(ma->masks_usage_stats, cpu);
>>> do {
>>> - start = u64_stats_fetch_begin_irq(&ma->syncp);
>>> - counter = usage_counters[i];
>>> - } while (u64_stats_fetch_retry_irq(&ma->syncp, start));
>>> + start = u64_stats_fetch_begin_irq(&stats->syncp);
>>> + counter = stats->usage_cntrs[i];
>>> + } while (u64_stats_fetch_retry_irq(&stats->syncp, start));
>>>
>>> ma->masks_usage_zero_cntr[i] += counter;
>>> }
>>> @@ -227,9 +227,10 @@ static struct mask_array *tbl_mask_array_alloc(int size)
>>> sizeof(struct sw_flow_mask *) *
>>> size);
>>>
>>> - new->masks_usage_cntr = __alloc_percpu(sizeof(u64) * size,
>>> - __alignof__(u64));
>>> - if (!new->masks_usage_cntr) {
>>> + new->masks_usage_stats = __alloc_percpu(sizeof(struct mask_array_stats) +
>>> + sizeof(u64) * size,
>>> + __alignof__(u64));
>>> + if (!new->masks_usage_stats) {
>>> kfree(new);
>>> return NULL;
>>> }
>>> @@ -732,7 +733,7 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
>>> u32 *n_cache_hit,
>>> u32 *index)
>>> {
>>> - u64 *usage_counters = this_cpu_ptr(ma->masks_usage_cntr);
>>> + struct mask_array_stats *stats = this_cpu_ptr(ma->masks_usage_stats);
>>> struct sw_flow *flow;
>>> struct sw_flow_mask *mask;
>>> int i;
>>> @@ -742,9 +743,9 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
>>> if (mask) {
>>> flow = masked_flow_lookup(ti, key, mask, n_mask_hit);
>>> if (flow) {
>>> - u64_stats_update_begin(&ma->syncp);
>>> - usage_counters[*index]++;
>>> - u64_stats_update_end(&ma->syncp);
>>> + u64_stats_update_begin(&stats->syncp);
>>> + stats->usage_cntrs[*index]++;
>>> + u64_stats_update_end(&stats->syncp);
>>> (*n_cache_hit)++;
>>> return flow;
>>> }
>>> @@ -763,9 +764,9 @@ static struct sw_flow *flow_lookup(struct flow_table *tbl,
>>> flow = masked_flow_lookup(ti, key, mask, n_mask_hit);
>>> if (flow) { /* Found */
>>> *index = i;
>>> - u64_stats_update_begin(&ma->syncp);
>>> - usage_counters[*index]++;
>>> - u64_stats_update_end(&ma->syncp);
>>> + u64_stats_update_begin(&stats->syncp);
>>> + stats->usage_cntrs[*index]++;
>>> + u64_stats_update_end(&stats->syncp);
>>> return flow;
>>> }
>>> }
>>> @@ -851,9 +852,17 @@ struct sw_flow *ovs_flow_tbl_lookup(struct flow_table *tbl,
>>> struct mask_array *ma = rcu_dereference_ovsl(tbl->mask_array);
>>> u32 __always_unused n_mask_hit;
>>> u32 __always_unused n_cache_hit;
>>> + struct sw_flow *flow;
>>> u32 index = 0;
>>>
>>> - return flow_lookup(tbl, ti, ma, key, &n_mask_hit, &n_cache_hit, &index);
>>> + /* This function gets called trough the netlink interface and therefore
>>> + * is preemptible. However, flow_lookup() function needs to be called
>>> + * with preemption disabled due to CPU specific variables.
>>
>> Is it possible to add some kind of assertion inside flow_lookup() to avoid
>> this kind of issues in the future?
>
> We could do something like WARN_ON_ONCE(preemptible()) but do not think such check should be added to the fast path.
I meant something like lockdep_assert_preemption_disabled(). This will not
impact fast path if CONFIG_PROVE_LOCKING disabled, but will allow to catch
issues during development.
>
>> It might be also good to update the comment for flow_lookup() function itself.
>
> Good idea, will do this in a v3
>
>>> + */
>>> + local_bh_disable();
>>> + flow = flow_lookup(tbl, ti, ma, key, &n_mask_hit, &n_cache_hit, &index);
>>> + local_bh_enable();
>>> + return flow;
>>> }
>>>
>>> struct sw_flow *ovs_flow_tbl_lookup_exact(struct flow_table *tbl,
>>> @@ -1109,7 +1118,6 @@ void ovs_flow_masks_rebalance(struct flow_table *table)
>>>
>>> for (i = 0; i < ma->max; i++) {
>>> struct sw_flow_mask *mask;
>>> - unsigned int start;
>>> int cpu;
>>>
>>> mask = rcu_dereference_ovsl(ma->masks[i]);
>>> @@ -1120,14 +1128,16 @@ void ovs_flow_masks_rebalance(struct flow_table *table)
>>> masks_and_count[i].counter = 0;
>>>
>>> for_each_possible_cpu(cpu) {
>>> - u64 *usage_counters = per_cpu_ptr(ma->masks_usage_cntr,
>>> - cpu);
>>> + struct mask_array_stats *stats;
>>> + unsigned int start;
>>> u64 counter;
>>>
>>> + stats = per_cpu_ptr(ma->masks_usage_stats, cpu);
>>> do {
>>> - start = u64_stats_fetch_begin_irq(&ma->syncp);
>>> - counter = usage_counters[i];
>>> - } while (u64_stats_fetch_retry_irq(&ma->syncp, start));
>>> + start = u64_stats_fetch_begin_irq(&stats->syncp);
>>> + counter = stats->usage_cntrs[i];
>>> + } while (u64_stats_fetch_retry_irq(&stats->syncp,
>>> + start));
>>>
>>> masks_and_count[i].counter += counter;
>>> }
>>> diff --git a/net/openvswitch/flow_table.h b/net/openvswitch/flow_table.h
>>> index 6e7d4ac59353..43144396e192 100644
>>> --- a/net/openvswitch/flow_table.h
>>> +++ b/net/openvswitch/flow_table.h
>>> @@ -38,12 +38,16 @@ struct mask_count {
>>> u64 counter;
>>> };
>>>
>>> +struct mask_array_stats {
>>> + struct u64_stats_sync syncp;
>>> + u64 usage_cntrs[];
>>> +};
>>> +
>>> struct mask_array {
>>> struct rcu_head rcu;
>>> int count, max;
>>> - u64 __percpu *masks_usage_cntr;
>>> + struct mask_array_stats __percpu *masks_usage_stats;
>>> u64 *masks_usage_zero_cntr;
>>> - struct u64_stats_sync syncp;
>>> struct sw_flow_mask __rcu *masks[];
>>> };
>>>
>>>
>>> _______________________________________________
>>> dev mailing list
>>> dev@...nvswitch.org
>>> https://mail.openvswitch.org/mailman/listinfo/ovs-dev
>>>
>
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